1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser printer and the like, which forms an electrostatic latent image on an image bearing member, or a photosensitive drum and supplies toner materials onto the electrostatic latent image through a developing roller in contact under pressure with the photosensitive drum to develop the image, and more particularly, to an image forming apparatus in which the developing roller can be released from the photosensitive drum after completion of image formation to achieve the printed images with high quality.
2. Description of Related Art
In conventional image forming apparatus such as a laser printer, generally, the surface of a photosensitive drum is charged uniformly by a scorotron type charger and the like and irradiated by an exposure device such as a laser beam scanner which projects a light image corresponding to an output image, thus forming an electrostatic latent image thereon. A developing roller supplies toner to the electrostatic latent image on the photosensitive drum to form a toner image. Then, the toner image is transferred onto a sheet of paper. This paper is heated by a fusing device that fuses the toner to form a printed image on the paper.
Such the image forming apparatus has the following disadvantages; when the photosensitive drum and the developing roller are kept in contact with each other for a long time after completion of printing, the substance extracted from a conductive rubber member defining the surface of the developing roller may stick to the surface of the photosensitive drum, resulting in the chemical deterioration of the photosensitive drum surface and the generation of strip-like stains (referred to as drum stains, hereinafter) which will deteriorate a printing quality.
To avoid the above disadvantages, there have been proposed various types of image forming apparatus. For example, Japanese Patent unexamined publication No. 2-37380, which has claimed priority from U.S. Pat. No. 4,891,674, discloses a development apparatus for use in an electrophotographic printing machine in which an applicator roller that has developer material adhering releasably to a surface thereof is mounted rotatably in a housing chamber, and racks are provided at an interval on the external surface of the side wall of the housing. The racks are engaged with pinion gears. Extension coil springs are mounted between the housing and a frame of the apparatus. When the applicator roller is driven by a motor to rotate in one direction, a rotation torque is transmitted to a constant torque slip clutch mounted on a shaft of the pinion gears. The housing is lifted up to the height against a constant load of the extension springs as the applicator roller is rotated, so that the applicator roller comes into contact under pressure to a photoconductive belt, operating to develop an electrostatic latent image. When the motor is de-energized, the housing is moved downward by the force of the extension springs exerting on the housing through the racks and pinions, bringing the applicator roller to a position spaced from the photoconductive belt. Thus, the photosensitive drum and the applicator roller can be supported spaced from each other after completion of printing.
However, the above conventional development apparatus is designed so that the housing is moved upward or downward to cause the applicator roller to come into contact with or become apart from the photoconductive belt. For the purpose, the gears which couple the applicator roller shaft to the pinion gear shaft, the constant torque slip clutch, the pinion gears, and the racks are used for a mechanism to transmit the power from the applicator roller to the housing. In addition, the extension coil spring is used to urge the housing to move in one direction. Accordingly, the mechanism to cause the applicator roller to come into contact with or be separated from the photoconductive belt is very large-sized, complex, and large in the number of components, resulting in an increase in cost.